A DC-DC converter is an electronic circuit adapted to convert a direct current source from a voltage value to another one. An important application field of DC-DC converters is digital electronic devices (for example, computers) that are supplied by a main power system. Particularly, a generic digital electronic device supplied by the main power system is provided with a supply circuit adapted to provide a DC voltage by rectifying an alternating voltage. A digital electronic device, however, is generally formed by a plurality of sub-circuits, with each of them needing a supply voltage value that is different (higher or lower) than the voltage provided by the supply circuit.
One of the most relevant among the known types of DC-DC converters is switching DC-DC converters. Considering a DC-DC converter of the “step-down” type, wherein the input voltage is converted into an output voltage having a lower value, an example of the operation thereof will be now described. During a first phase—denoted as a “main phase”—the switching DC-DC converter electrically couples a source of the input voltage (to be converted) to a terminal of a reactive element, typically an inductor. As a consequence, in this phase the reactive element stores magnetic energy from an input current from the input voltage source. In this phase, the output load requiring the (converted) output voltage receives energy from the input voltage source. In a second phase—denoted “secondary phase”—the reactive element is disconnected from the input voltage source, and the output load receives the magnetic energy that has been stored in the reactive element during the main phase. Acting on the ratio between the main phase and secondary phase durations—denoted “duty cycle”—, it is possible to regulate the transfer of energy from the voltage source to the output load in a controlled way, and thus bring the voltage provided to the output load to the requested value.
Practically, a switching DC-DC converter of such type may be implemented by connecting a first terminal of the inductor to two switch circuits (for example, power MOS transistors), and particularly a first switch—denoted main switch—adapted to electrically couple the first terminal of the inductor to the source of the input voltage during the main phase, and a second switch—denoted secondary switch—adapted to electrically couple the first terminal of the inductor to a terminal providing a reference voltage (for example, ground) during the secondary phase, and connecting the second terminal of the inductor to the output load (for example, through a voltage stabilizer). The two switch circuits are driven by respective driving signals, for example by means of complementary versions of a same square signal having a constant frequency (this particular case is referred to as constant frequency driving). By varying the duty cycle of such driving signal in such a way to increase/decrease the duration of the main phase with respect to the secondary phase one, it is possible to increase/decrease the value of the output voltage obtainable by means of the conversion. Since the current request from the output load typically varies during operation, for keeping the output voltage value as stable as possible, the switching DC-DC converters are typically provided with a feedback control circuit, adapted to sense any difference of the output voltage from a desired value and vary the duty cycle of the driving signal for compensating and counterbalancing such differences.
Generally, the efficiency of the switching DC-DC converters is afflicted by two main types of losses, i.e., the so-called conduction losses and the so-called switching losses.
The conduction losses are due to the power dissipation that generates when the current flows across the parasitic resistances of the devices (mainly, the resistances of the switches and of the inductor). Such losses are proportional both to the amount of such resistance values and to the square of the current crossing the devices.
The switching losses are instead due to the consumption of the electric power required for the switching of the switches. Such losses are proportional to the switching frequency of the switches, and thus to the frequency of the driving signal, and in some cases may be particularly high, for example in case it is necessary to provide the charge required for turning on a switch implemented with a power MOS transistor that exhibits a high drain-to-source voltage—in jargon, “hard switching” condition.
Switching DC-DC converters are characterized by a great efficiency for loads that require currents of high values, while such efficiency falls when the currents required by the load become very low, near to zero (light load condition).
Indeed, the current of the inductor of a switching DC-DC converter exhibits an oscillatory trend, and particularly an increasing trend during the main phase and a decreasing one during the secondary phase, both having a slope which depends on the inductance value of the inductor and on the difference between the converter input voltage and the converter output voltage. The mean value of the inductor current of the DC-DC converter has instead a value that corresponds to the value of the current required by the load. As a consequence, considering that the amplitude of such oscillations is independent from the value of the current required by the load, if such current is close to zero, the power transfer balance becomes unfavorable, because the inductor current becomes negative for certain time intervals (during both the main phase, and the secondary phase).
As a consequence, in a light load condition, wherein the power transfer balance is already highly unfavorable, the power dissipated because of the switching losses may even exceed the power actually transferred from the source to the load and improved control systems for such operating conditions are needed.